The invention relates generally to photovoltaic (PV) systems and more particularly to a system and method of implementing intra-module dc grounding for a system of PV ac modules.
Nearly all electrical systems in the U.S. are grounded to mitigate the impacts of lightning, line surges, or unintentional contact with high voltage lines. Most PV systems include modules with metal frames and metal mounting racks that are in exposed locations, e.g. rooftops where they are subject to lightning strikes, or are located near high voltage transmission lines that in the event of high winds, etc., can come into contact with PV arrays.
The modules in a typical PV array have aluminum frames that are often anodized. The 2008-NEC code that has the same requirements as the draft 2010-NEC code and governs installation of PV systems requires exposed metal surfaces be grounded. There are special dc wiring and grounding requirements that must be met specifically for dc module strings that can produce voltages at high as 600 volts. A failure in the insulating material of the PV laminate could allow the frame to be energized up to 600V dc.
The installer of a PV system is required to ground each module frame per the NEC code and UL standard 1703. This inter-module grounding must be met using a heavy, e.g. at least #10 gauge) copper wire and a 10-32 screw that can cut into the frame. Additional assurances are required even for frames having anodized surfaces. Washer/connectors in such cases are used to cut into the metal frame and provide the best electrical contact. These processes require additional components for installation and require a substantial level of effort to install mounting brackets and grounding wires.
Grounding continuity must also be addressed per the NEC code. The oldest NEC requirement necessitates making the ground connection first and breaking the ground connection last. Not all installations follow this practice. The circuit conductors should never be connected without a solid ground in place. GFI cannot prevent shock in this situation.
FIG. 1 illustrates a system of PV dc-voltage modules 10 configured to generate a desired dc output voltage that is known in the prior art. The system of PV dc-voltage modules 10 is a series configuration capable of generating up to 600V dc. Thus, a failure in the insulating material of the PV laminate could allow the corresponding frames to be energized up to 600V dc. A PV system installer is required to ground each module frame 12 per NEC code and UL standard 1703 as stated herein. Such frame-to-frame grounding is typically accomplished using a heavy gauge copper wire and a 10-32 screw that can cut into the frame(s). Further, washers/connectors are employed to ensure that reliable electrical contact is made and maintained, even for frames having anodized surfaces.
FIG. 2 illustrates a system of PV ac-voltage modules 20 configured to generate a desired ac output voltage that is known in the prior art. Each ac-voltage module 22 comprises a dc-ac micro-inverter 24 integrated therewith to generate the desired ac output voltage. The availability of such compact and efficient micro-inverters 24 and their integration with individual dc-voltage PV modules 26 has led to the commercial realization of the ac module 22 which produces a 120V ac or 240V ac output. The 2005 NEC code section 960.6 addresses the ac module. Because all of the dc wiring requirements are now considered to be integral to the ac module and are not accessible, dc requirements in the NEC code are no longer applicable and installation is simplified in that there is no longer a requirement to provide a physical dc disconnect switch.
Presently, all commercial systems that employ micro-inverters 24 still require an equipment ground, meaning that all modules with metallic frames 28 and metal mounting systems have to be connected to a common earth ground through a low resistance path. Such inter-module ground connections are still be made using processes that require the use of metallic splices, lugs, penetrating washers, and wires. All of these methods require hands-in grounding connections be made at the time of installation and usually requires the presence of an experience electrician.
In view of the foregoing, it would be advantageous to provide a system and method for implementing intra-module dc grounding for a system of PV ac modules, without necessitating use of metallic splices, screws, lugs, penetrating washers, brackets, wires and the like.